1. Field of the Invention
This invention relates to nuclear energy generating devices, specifically to a hardware module for confining and heating a plasma inside a nuclear energy device.
2. Prior Art
Scientific Background for Nuclear Power Generation
Fossil fuels burn by recombining chemical elements into new molecules. Nuclear fuels “burn”, in the sense of recombining the particles in nuclei into new nuclei.
Nuclear fuels have certain advantages over fossil fuels. Burning nuclear fuel does not produce carbon dioxide. Nuclear fuel is cost effective to store and transport.
Nuclear energy devices are either fission devices or fusion devices. A “nuclear reactor” is commonly a fission type power plant. Uranium or plutonium is input to the reactor as fuel. Neutrons induce the fuel nuclei to fission, in other words, to break apart. Each nucleus that fissions liberates a large amount of heat energy. Many millions of fission reactions happen each second to produce megawatts of heat. Heat energy is converted into electric power by turbines and generators.
Nuclear reactors produce harmful chemical wastes. The danger of a meltdown, such as at Three Mile Island or Chernobyl, is always a worry. For these reasons, nuclear reactors have not competed successfully with fossil fuel power plants.
Fusion has potential advantages over fission. Fusion energy devices do not produce toxic wastes and do not melt down. Fusion reactors burn plentiful elements, such as hydrogen or boron. Pairs of these light elements fuse, in other words join together, to form new nuclei and possibly also neutrons. As a by-product of the fusion reaction a relatively large amount of useable energy is released. Common fusion energy devices utilize a plasma to supply the fuel to be fused. A plasma is a gaseous cloud of charged particles, including electrons and/or ions. Ions are the nuclei of fuel atoms which have been stripped of their electrons. The fusion energy device confines the hot plasma inside a vacuum tank.
Energy flows out of the plasma and is converted to electric power. The plasma must be heated to a high temperature, be confined for a long time, and be maintained at a high density. These 3 measures of the confined plasma, 1) temperature, 2) confinement-time, and 3) density, when multiplied together determine the rate of power output. The higher the power output of a plasma energy device, the better the device will compete with fossil-fueled power plants.
The tokamak is the most common fusion energy device on earth. A hot plasma is confined by electromagnets inside a large donut-shaped vacuum tank. Tokamaks were invented in the 1950's in Russia. Powerful electromagnets steer ions away from the vacuum tank walls. At high density and temperature, the loss rate of ions becomes excessive. Replacing these lost ions consumes more power than the device produces from fusion. The power-in to power-out balance never reaches the break-even point. Efforts to solve this problem have led to constructing bigger and more expensive prototypes. The most recent tokamak prototype is currently ITER, being constructed in France. ITER will cost $10 billion to build and operate for 30 years. ITER will still be only a prototype. It is not expected to produce any useable amount of power.
Inertial Electrostatic Confinement of Plasmas—Bussard's Fusor
A promising alternative to the tokamak is disclosed in U.S. Pat. No. 4,826,646(1989) to Robert W. Bussard. This patent describes a plasma energy device which uses the principle of inertial-confinement. The ions of the plasma are confined in an electric field which is approximately spherical(quasi-spherical). The force on each ion points inward toward the center of a quasi-spherical vacuum tank. All the ions travel in and out along radial lines, converging from all directions. The ions are kept away from the outside walls of the vacuum tank by the electric field. At the center, the ions come close together with maximum velocity. Each ion makes many thousands of passes through the center of the sphere until it finally hits another ion and fuses. Fusion energy flows out to the walls of the vacuum tank.
Inertial-confinement is a subclass of plasma fusion energy devices. The device disclosed by Bussard—1989, cited above, is an important example. Another important example is electrostatic confinement developed by Thomas McGuire in his 2007 M.I.T. PhD. thesis entitled “Improved Lifetimes and Synchronization Behavior in Multi-grid Inertial Electrostatic Confinement Fusion Devices”. McGuire—2007 analyzes data from prototype testing of an inertial-confinement design originated at M.I.T. The M.I.T. device builds on U.S. Pat. No. 3,664,920(1972) to Robert L. Hirsch. The Hirsch device utilized a spherical grid of fine wires to keep the ions away from the walls of the vacuum tank. However, the wires themselves intercepted too many of the circulating ions. The density and confinement-time in the Hirsch—1972 device never reached high enough values to make practical fusion energy.
In the following discussion the term “fusor” is used to refer to any quasi-spherical plasma fusion energy device based on the principle of inertial-confinement. Patents to Bussard—1989 and Hirsch—1972 disclose the two main examples. Bussard's device also received a trademark “Polywell”, abandoned Jun. 18, 1992.
The electric field in the Hirsch—1972 fusor was designed to point directly toward the center of the sphere at all points inside the grid. The M.I.T. fusor replaced Hirsch—1972's spherical grid of wires by multiple concentric grids of wires. Multiple grids added a transverse component to the radial electric field of Hirsch—1972. The extra field component was designed to pull the ions back from hitting the wires. However, attempts to raise the plasma density to practical levels again resulted in a loss of ions.
The cause of the loss was analyzed by McGuire, using a commercial computer simulation program. The computer tracked the positions of millions of simulated ions as they traveled back and forth through the center of the fusor. Simulated ion density was gradually raised by adding more and more ions to the simulation. When the density reached a certain critical value, the ions formed clumps. This clumping order McGuire called “synchronization” in his thesis title. Because clumps had more inertia than individual ions, the electric field could not contain them. Many clumps hit the walls and this loss of ions prevented the density from going higher. McGuire concludes as follows: “Thus, while a low density [fusor] device may theoretically operate at energy break-even or better, there are serious problems with scaling up the density to reach useful reaction rates and powers.”
“Break-even” operation is an important goal to prove a fusor works. At break-even a fusor consumes the same amount of power as it produces. Break-even performance is only a short development step away from practical power generation.
McGuire's effort to improve on Hirsch—1972 was unsuccessful. McGuire did prove the value of Particle-in-cell(PIC) simulation as a virtual reduction-to-practice technique. This same technique has been used in designing the apparatus of this patent.
The Bussard fusor differs from the McGuire/Hirsch fusor in two major ways: 1) Bussard's plasma is an almost-neutral mixture of electrons and ions, instead of pure ions as in McGuire/Hirsch; and 2) Bussard's electric field is formed by excess electrons, instead of grid wires. These differences give Bussard—1989 an advantage over McGuire—2007 and Hirsch—1972. This invention improves on Bussard—1989.
The use of a quasi-neutral plasma reduces the electric energy stored in the plasma. Otherwise repulsion of like charges forces the ions apart. In McGuire/Hirsch, the ions tend to run for the walls and hit the wires.
In Bussard—1989 there are no grid wires to impede the flow of ions. The ions return again and again to the center, where they eventually fuse. Input power is spent on supplying extra electrons to replace ones that get lost at the tank walls. Lost electrons require much less energy to replace than lost ions would require. Electrons insulate the ions from seeing the tank walls. This two-layer (electron-ion) feature of Bussard—1989 results in longer ion confinement times and lower input power.
Prior Attempts to Build a Working Scale Model of Bussard's Fusor
From 1989 to 2007, Dr. Bussard worked continuously to try to prove his 1989-invention. He died in 2007, at the age of 89. Prototype testing is still the fulltime occupation of the EMC2 Fusion Development Corporation of Santa Fe, founded by Bussard. Bussard's work from 1989 through 2006 has been continuously reported in many scientific-journal publications and 100 Corporation-internal reports, many of which are 30-plus pages long. The most recent of the publications and the most recent of the internal reports have been selected for analysis here. Bussard's 2006—IAC publication, “The Advent of Clean Nuclear Fusion: Super-Performance Space Power and Propulsion”, can be viewed on the Corporate website, emc2fusion.org. This publication shows individual photos of 7 scale-models of various embodiments of Bussard's invention. These models were developed over 11 years of research sponsored by the United States Navy. None of these models made any useful fusion power.
The last and most successful of Bussard's scale models was called WB-6. WB stands for WiffleBall. The toy WiffleBall has a similar topological shape to the confining magnetic field of the fusor. WB-6 was reported in 2006—IAC as making measurable fusion. The publication states that 3 neutrons were detected in one test in November, 2005. During this test one of the magnet wires in the fusor melted before the test was completed. No further neutron detections have been reported by the EMC2 Fusion Corporation.
The final test of WB-6 used deuterium(D) fuel. D+D fusion produces a neutron half the time. By measuring the rate at which neutrons come out of the fusor, the experimenter computed the rate of fusions occurring in the center of the device. This rate-computation included a factor of 2 in order to account for D+D fusion which does not make a neutron. The computation also took into account inefficiency of neutron detection, detector solid-angle, and other factors familiar to one skilled in the science of neutron detection.
The neutron detectors might have counted electronic noise and cosmic rays which can cause fake counts. It is usual practice to repeat the measurement without the neutrons. Turning off the neutrons could have been accomplished by replacing the deuterium-ions by hydrogen-ions, etc. Spurious counts recorded under such background conditions should be subtracted from the counts in the real experiment. The fusion rate might even be zero if all 3 counts turn out to be background.
Additional details on the neutron detectors were searched for by downloading Bussard's 2006—EMC2 internal Corporation report “EMC2 Inertial-Electrostatic Fusion (IEF) Development: Final Successful Tests of WB-6; October/November 2005” from website http://en.wikipedia.org/wiki/Polywell. On pg. 24 Bussard writes the following: “The last count (or perhaps any of the 3 counts) is possibly able to be dismissed as noise . . . . The flaws in WB6 are fairly obvious: lack of cooling, tight bends on the magnet wiring, very short duration tests, with limited ability to monitor what happens, lack of diagnostics, etc.” Notably, Bussard does not use the word “success” anywhere in the Summary or Conclusions sections of 2006—EMC2. Neutron background measurements are not mentioned in the report. The lack of background subtraction casts doubts on the reported testing of the WB-6 prototype.
The Future of the Bussard Fusor
Dr. Bussard's 1989 invention is still very promising. The problems described above show the difficulties of building a working model fusor. The Bussard—1989 patent was largely theoretical. It lacked practical details needed to build a working model. Scale models are expensive to build; therefore, only a few could be tested. The next prototype, WB-7, is costing the Corporation $1.75 million to build and test in 2008.
Simulating the performance of a fusor in the computer has the advantages of being fast and inexpensive compared to model construction. Bussard's main simulation program is described briefly in 2006—IAC. A more complete description is from an EMC2—1991 Corporate report: “The EKXL code . . . is a 1-D radially-dependent Poisson-solver.” Because EKXL is only one-dimensional(1-D), it only applies to fusors which are perfect spheres. But the real fusor is in the shape of a cube, not a sphere. FIG. 15 of Bussard's 2006—IAC shows a graph of simulated ion-density. The density at the center of the fusor is 1000 times larger than the density at the outside edge of the fusor.
The simulations for this patent utilized a 2-D PIC program called OOPIC Pro. 2-D (i.e. 2 spatial dimensions) is required to make a realistic simulation of a cube. OOPIC Pro predicted central density equal to edge density, not 1000 times larger as reported in EKXL. This vast decrease in predicted density would result in an even larger decrease in fusor performance, other things being equal.
OOPIC Pro, from the Tech-X Corporation of Boulder, Colo. (website txcorp.com), was developed over 30 years of work by Professor Charles Birdsall and his group at the University of California, Berkeley. The technique Birdsall and co-workers used to simulate plasma physics effects was called Particle-in-cell simulation(PIC). Birdsall's simulation software has been successfully used to predict the performance of many different types of plasma devices, from vacuum tubes to tokamaks. The OOPIC-Pro software was recently updated from the Berkeley code to use object-oriented(OO) C-source language. OOPIC Pro is well-suited to run on desktop workstations, such as the 2.5 Ghz Pentium-4 by Intel.
PIC simulation techniques were also used by Bussard as described on pg. 7 of 2006—IAC: “Device and system operation and performance at startup conditions, at very early times, have been modeled by complex electrostatic computer codes that determine the coulombic interactions between all particles throughout the system and plot trajectories and densities in the system.” The results of Bussard's PIC simulations were not made public, except for the brief quote just given. Bussard apparently switched to EKXL as his simulator of choice.
OOPIC Pro is fully 2-dimensional in spatial variables and 3-dimensional in velocities. It was developed entirely independently from the software used by Bussard. OOPIC Pro uses an electromagnetic field solver to solve Maxwell's equations. Both simulation codes used by Bussard solved simplified(i.e. “electrostatic”) forms of Maxwell's equations. This is an inaccurate simplification for the high ion-densities needed for fusion. A 2-D “electromagnetic” solver, like in OOPIC Pro, is required to correctly include diamagnetic effects. Without diamagnetic effects, the predicted ion confinement time would be incorrect.
Use of a more realistic computer simulation has led to an improved design for a prototype over WB-7. In addition, the improved simulation points the way to the next generation prototype fusor, the steady-state fusor. WB-6 only operated for a fraction of a millisecond before it shut down and eventually melted. WB-7 is being built to run longer, but still only a fraction of a second. The reason that WB-6 and WB-7 were only designed to run for a short time is to save money. Short-pulsed operation is still costing EMC2 Corporation $1.75 million.
A practical nuclear power fusor must run continuously for days and months.
Drawbacks in the Prior Art Preventing Continuous (i.e. Steady-State) Testing
FIG. 1 shows a drawing of WB-6 copied from FIG. 12 of 2006—IAC. WB-7 is a close copy of WB-6. Both WB-6 and WB-7 were designed to run only in pulsed mode, i.e. not steady-state. Six identical electromagnets are mounted, in donut-shaped vacuum containers 100, on the six faces of a cube. The five upper magnets are supported by welded corner posts 102 from one to the next. The bottom magnet is supported by four ceramic legs 104 onto a metal plate, not shown. The metal plate(not shown) is supported by four similar legs onto the floor of a vacuum tank.
The following limitations of WB-6 are evident in FIG. 1 and from the preceding discussion:
(a) The welded assembly of the corner posts 102 make the magnets difficult to service if one needs repair. To repair a magnet requires cutting off its four welded corner posts 102 and breaking the wires they contain. Such a repair is impractical. WB-6 was abandoned as soon as a magnet winding shorted and was never repaired.
(b) The magnets' corner posts 102 are located in a crucial crossroad for flowing electrons. Electrons will hit these posts 102 and be lost. Lost electrons must be replaced as fast as they are lost. Replacing lost electrons requires increased electron input power.
(c) The magnets have no cooling lines connected to them. About one second after turning the magnet power on, the magnets will overheat. Overheating limits the maximum length of a pulsed test to one second. If the magnet power were accidentally left on, the magnets would melt.
(d) The magnets' donut-shaped vacuum containers are round when viewed in cross-section. Commercial companies sell only rectangular cross-section magnets with water cooling. Simulations for this patent show these commercial magnets will work as well as the un-cooled round ones in WB-6. Handmade magnets are expensive compared to commercially available magnets. Commercial magnets are less expensive and more reliable than hand-made ones.
(e) A 2 m diameter vacuum tank encloses a 0.3 m diameter WB-6 device. Vacuum is expensive to create and maintain. A smaller tank size, about twice the cube dimension, will work as well. Reducing the size of the vacuum tank saves money.
(f) Building a fusor around a dodecahedron would improve central density. A 1000× concentration of ions at center of a fusor was erroneously predicted by EKXL. A higher dimension polyhedron, like the soccer ball with 32 faces, would have still better central density. Each face of the polyhedron needs a separate electromagnet coil to cover it. The cube needs six; the dodecahedron would need twelve; the soccer ball, thirty-two. Interchangeable magnets are needed for mass-production, not welded into position as with WB-6.